Generally, a heat-resistant material to be activated at a high temperature is protected from a high-temperature corrosive environment by forming and maintaining a protective scale, such as Cr2O3, Al2O3 or SiO2. This scale causes deterioration in mechanical characteristics of the heat-resistant material, and thereby an element for forming the protective scale, such as Cr, Al and/or Si, cannot be practically added in a sufficient amount.
Currently, a film containing a high concentration of Cr, Al and/or Si is formed on a surface of a heat-resistant material through various processes. For example, a Ni-based superalloy for use in gas turbines, jet engines, etc., has an Al or Cr diffusion film formed through a pack cementation process, a CVD process, etc., or a MCrAlY film formed through a thermal spraying process, an EBPVC process, etc.
Among the above film forming processes, while the thermal spraying process is capable of forming a film onto a large area at low cost, it can be applied only to a member having a relatively simple configuration. While a sputtering process or a PVD process is capable of accurately forming a film, it has restrictions on size and productivity and leads to increase in cost.
Further, these processes have difficulty in forming a film in a through-hole or a gap. While the CVD process or the pack cementation process designed to supply an element for forming a film, or a film-forming element, in the form of gas is capable of forming a film even onto a member having a complicated configuration and in a through-hole or a gap, it is based on atmosphere control. Thus, the CVD process or the pack cementation process is restricted in size of a target member, and inferior in productivity and cost performance. Moreover, a film is generally formed over the entire surface of the member, or it is difficult to selectively form a film only in a specific region of the member.
A film forming process capable of solving the above characteristics and disadvantages includes a plating process. The plating process is typically designed to electrochemically deposit a film from an aqueous solution, and theoretically capable of forming a film onto a region allowing an electrolytic solution to enter thereinto. Further, a nonaqueous solution or a molten salt can be used in the plating process to form a film of a base metal, such as Al or Mg. In addition, a surface of a target material can be masked to selectively form a film only in a specific region of the surface.
However, the plating process has restrictions on a combination and composition control of film-forming elements, and an obtained product is generally required to be subjected to a heat treatment at a high temperature so as to ensure adhesion of the formed film.
A member to be used at a high temperature is not exposed to the high temperature in its entirety. For example, in a sheath of a thermocouple, while the edge of the sheath is exposed to high-temperature combustion gas, a large portion of the sheath is maintained in a low temperature range. Further, if a film is formed on the entire surface of the sheath, the film formed on a specific region, such as a mounting region or a connection region, has to be removed in a subsequent process. Typically, these specific regions are maintained at a low temperature during use.
Thus, it is unnecessary to form a film over the entire surface of a member, and it is rather desired to selectively form a film only in a specific region to be exposed to a high-temperature corrosive circumstance.
In a process for forming a film excellent in high-temperature corrosion resistance in a specific region having a complicated configuration, the existing film forming processes have the following advantages and disadvantages. The thermal spraying process, the PVD process or the sputtering process can form a film onto a specific region by use of a masking technique. However, these processes cannot form a film on a region in a through-hole or a gap. The CVD process or the pack cementation process can form a film onto a region in a through-hole or a gap. However, these processes have restrictions on film formation only in a specific region. The plating process can form a film on a region in a through-hole or a gap and on a specific region by use of a masking technique. However, a product from the plating process is generally required to be subjected to an after-heat treatment for ensuring the adhesion between a base material and a formed film. Moreover, the plating process has restrictions on a type and composition control of film-forming elements.
Thus, there is the need for developing a process of forming a high-temperature corrosion-resistant film having an “intended configuration” in a “desired region” of a heat-resistant material at “low cost” with “high productivity”.